The 24-year-old was a main-stay of Brentford’s promotion challenge last season, playing 38 Sky Bet Championship matches and both play-off games against Middlesbrough.
Dallas made his debut for Northern Ireland in 2011, in a 2-0 defeat to Wales, but had to wait until 2015 for his next four caps.
The midfielder was plucked by Mark Warburton from Crusaders in Northern Ireland in 2012 but did not set Griffin Park alight at first.
Following a loan to Northampton Town in late 2013, however, turned the winger into an integral figure at Brentford.
Dallas scored eight goals in all competitions for the Bees last term.
Meanwhile Nicky Ajose may not be on his way out of Elland Road – Sky sources say Swindon Town have failed in a loan bid for the striker.
The former Manchester United trainee spent last season on loan at Crewe Alexandra, scoring eight goals in 27 appearances.
In fact Ajose has been on loan at six different clubs over his five-year career, with other spells at Scunthorpe, Chesterfield, Crawley, Swindon, and two separate stints at Bury.
Having scored eight in 11 games between December and February last year at Crewe, Ajose then went 15 matches without a goal.
Swindon will be looking for a back-up to current forwards Jonathan Obika and Michael Smith, who scored 28 goals between them last year.